1. Field of the Invention
The present invention relates to fluorescent lamps and, in particular, to a long life, high efficiency fluorescent lighting system in which the phosphor is coated on a surface external to the lamp and an ultraviolet reflective layer is used to prevent hazardous ultraviolet rays from escaping the light source.
2. Description of Related Art
Existing fluorescent lamps have limitations in performance and lifetime that are undesirable. A typical mercury vapor (Hg) fluorescent lamp includes a phosphor coating on the inside surface of a glass tube. When the Hg vapor is ionized inside the tube, the lamp discharge emits radiation, including ultraviolet, that in converted to visible light by the phosphor coating.
The performance of a standard fluorescent lamp suffers from several shortcomings inherent in its basic design. In particular, the phosphor coating on the inside surface of a fluorescent tube is exposed to heat and mercury. This exposure can degrade or poison the phosphors. Furthermore, where filaments are used to provide power to the lamp, the filaments can evaporate. This leads to a reduction in light output, and ultimately filament and lamp failure.
Using a thicker phosphor layer can extend the life of the phosphor coating. However, a thick phosphor layer reflects light much better than it transmits light. Thus, in applying the phosphor coating to the inside of a fluorescent tube, there is a trade-off between a thin coating that transmits light more efficiently versus a thick coating that provides a longer lifetime.
Filament evaporation and phosphor poisoning are two failure modes for prior art fluorescent lamps. An invention that addresses these failure modes would work to extend the lifetime of the lamp. In a system including such a lamp, the lamp ballast would become the component with, potentially, the shortest lifetime and therefore the most likely component to require replacement. It would be beneficial, therefore, for the task of ballast replacement to be made simple.
The invention comprises a fluorescent lamp system wherein an ultraviolet producing discharge lamp contains no phosphor internal to the tube. Rather a cover, housing or sleeve physically distinct from the lamp includes a phosphor. The cover or sleeve further includes an ultraviolet filter, preferably in the form of an ultraviolet reflective/visible light transmissive layer positioned to reflect ultraviolet energy that is not converted to visible light on a previous pass through the phosphor, back toward the phosphor. It has many embodiments. The preferred embodiments are described below.